1. Field of the Invention
This invention relates to an audio multiplex television tuner for receiving audio multiplex television broadcasts, which is suitable, for instance, for vehicles.
2. Background Art
A conventional sound multiplex television tuner of this type is as shown in FIG. 1. The tuner is connected to an antenna 1 and includes a front end 2, a video intermediate frequency amplifier (PIF) 3, a video detector 4, a frequency conversion circuit 5, a sound intermediate frequency amplifier circuit (SIF) 6, a sound detector 7, a sound multiplex sub-signal demodulation circuit 8, and de-emphasis circuits 9m and 9s. A Q signal detector circuit 10 identifies control signals representing broadcast modes, namely, a stereo broadcast or a bilingual broadcast. A mode switching circuit 11 provides a mode switching signal in response to the control signal identifying operation of the detector circuit 10. A mode designation input to the mode switching circuit 11 can be manually set to override the stereo capability or to select an alternate mode, such as the bilingual broadcast. The output of the mode switch 11 controls a matrix circuit 12, receiving outputs of the deemphasis circuits 9m and 9s.
In the television tuner thus organized, a main signal (L+R), which is provided at the output terminal of the second detector 7, is applied through the deemphasis circuit 9m to the matrix circuit 12, while a subsignal, which is provided at the output terminal of the sub-signal demodulation circuit 8, is supplied through the other de-emphasis circuit 9s to the matrix circuit 12. As a result of which, the matrix circuit 12 outputs sound signals separated into the right and left channels.
Heretofore, FM stereo tuners have extensively employed a noise suppression circuit which changes the separation according to the field strength of a signal received. Applying the noise suppression circuit to the sound multiplex television tuner, as it is, causes no trouble in a stereo mode. However, in a bilingual mode or an alternate sound mode such as an SAP (Separate Audio Program) mode in the U.S.A., the tuner may output only the sub-signal. If only the sub-signal is outputted, the person will experience a strong sense of incongruity. Thus, in the case of a bilingual mode or an alternate sound mode, it is not practical to apply the noise suppression circuit to the sound multiplex television tuner as it is.
The present inventors have filed a U.S. patent application, Ser. No. 726,753 on Apr. 24, 1985, incorporated herein by reference. In that application, the inventors showed in FIG. 3 a prior art sound multiplex tuner. In FIG. 2 of the present application is shown a tuner circuit similar to that of Ser. No. 726,753. Therefore, further discussion of FIG. 2 will be omitted. This tuner receives a multiplexed broadcast according to the U.S. method.